1. Field of the Invention
The present invention relates to a vacuum forming machine for attaching a decorative sheet onto a base member under a reduced pressure and also relates to a method of making such a vacuum formed product.
2. Description of the Related Art
Recently, it was proposed, as an alternative decorating technique to painting, that a decorative sheet be attached to the surface of a base member. A base member decorated with a decorative sheet can be recycled more easily than a painted one. In addition, the decorated member can have a different type of fine appearance from that of a painted one, thus contributing to improving its decorativeness, too.
FIG. 17 illustrates an example of a decorative sheet. The decorative sheet 10 shown in FIG. 17 includes a decoration layer 1 and a supporting layer 2 that supports the decoration layer 1. Examples of the decoration layer 1 include an ink layer that has been formed by printing and a metal layer that has been formed by depositing a metallic material. The supporting layer 2 is made of a resin material such as polyvinyl chloride (PVC). When the decorative sheet 10 is attached onto the base member, an adhesive is applied onto the surface of the decorative sheet 10.
Such a decorative sheet 10 is attached onto a base member while the decorative sheet 10 is being formed under a reduced pressure so as to trace the surface shape of the base member. A vacuum forming machine for forming and attaching a decorative sheet in this manner is disclosed in Japanese Patent Application Laid-Open Publications No. 2006-7422 and No. 63-214424, for example.
FIG. 18 illustrates a vacuum forming machine (reduced pressure coating system) 500 as disclosed in Japanese Patent Application Laid-Open Publication No. 2006-7422. The vacuum forming machine 500 includes a chamber box 521, in which the internal pressure can be increased and decreased adaptively. The chamber box 521 consists of an upper chamber box 522 and a lower chamber box 524.
Inside the upper chamber box 522, a heater 528 is arranged for heating a decorative sheet (skin member) 10. On the other hand, a table 509 for mounting a base member (core member) 16 is arranged inside the lower chamber box 524. The upper and lower chamber boxes 522 and 524 are connected to a vacuum tank 507 and a compressed air tank 508.
Now, it will be described how to attach the decorative sheet 10 using such a vacuum forming machine 500.
First of all, as shown in FIG. 18, the base member 16 is mounted on the table 509 inside the lower chamber box 524 and then the decorative sheet 10 is put on the upper surface of the lower chamber box 524.
Next, as shown in FIG. 19, the upper chamber box 522 is lowered, thereby bringing the upper and lower chamber boxes 522 and 524 into contact with each other with the decorative sheet 10 sandwiched between them. As a result, the upper and lower chamber boxes 522 and 524 are closed airtight.
Subsequently, as shown in FIG. 20, the upper and lower chamber boxes 522 and 524 are evacuated by establishing communication with the vacuum tank 507, thereby creating a reduced pressure state (a near-vacuum very low pressure state) inside the upper and lower chamber boxes 522 and 524.
Thereafter, as shown in FIG. 21, the decorative sheet 10 is heated with the heater 528. This heating process is carried out to the point that the decorative sheet 10 softens so much that the adhesive applied on the surface of the decorative sheet 10 has a sufficient degree of adhesiveness.
Then, as shown in FIG. 22, the table 509 inside the lower chamber box 524 is raised, thereby bringing the base member 16 into contact with the decorative sheet 10.
Next, as shown in FIG. 23, the pressure inside the upper chamber box 522 is restored to the atmospheric pressure (with the reduced pressure still maintained in the lower chamber box 524), thereby causing the decorative sheet 10 to be pressed against the base member 16 and formed so as to trace the surface shape of the base member 16 by taking advantage of the pressure difference between the upper and lower chamber boxes 522 and 524.
In this process step, if compressed air is introduced into the upper chamber box 522 by making the upper chamber box 522 communicate to the compressed air tank 508, the decorative sheet 10 is pressed more strongly against the base member 16.
Finally, as shown in FIG. 25, after the pressure inside the lower chamber box 524 is also restored to the atmospheric pressure, the upper chamber box 522 is raised and the base member 16 coated with the decorative sheet 10 is unloaded. In this manner, a vacuum formed product consisting of the base member 16 and the decorative sheet 10 attached to its surface can be obtained.
By performing such a vacuum forming process, even a base member with a great degree of surface unevenness can be decorated easily with a beautiful sheet.
In the vacuum forming machine 500 disclosed in Japanese Patent Application Laid-Open Publication No. 2006-7422, to prevent a hollow base member 16 from expanding and bursting or collapsing under the rising pressure inside the chamber box 521, there is a communication hole 504 that maintains communication between the inside of the base member 16 and the upper chamber 522. If the base member 16 is not a hollow one, however, a normal vacuum forming process may be carried out without using such a communication hole 504.
With the vacuum forming machine 500 disclosed in Japanese Patent Application Laid-Open Publication No. 2006-7422, however, when heated, the decorative sheet 10 will soften and sag down as indicated by the dashed curve in FIG. 21. If the forming process were carried out with the decorative sheet 10 sagged in this manner, then the decorative sheet 10 attached would slightly shift from its intended position, thus ruining its appearance (e.g., misaligning the pattern of the decoration layer 1). A similar problem will arise even when a vacuum forming machine (which is a picture applier) as disclosed in Japanese Patent Application Laid-Open Publication No. 63-214424 is used.